1. Field of the Invention
The present invention relates to a monolithic ceramic electronic component and, more particularly, to a monolithic ceramic electronic component in which an external terminal electrode is not only connected to exposed ends of internal electrodes but also connected to exposed ends of dummy conductors to thereby improve the fixing strength of the external terminal electrode.
2. Description of the Related Art
In recent years, miniaturization of electronic apparatuses, such as a cellular phone, a notebook computer, a digital still camera and a digital audio instrument, has progressed, and these electronic apparatuses use a large number of monolithic ceramic electronic components that enable miniaturization and provide high performance.
Generally, each monolithic ceramic electronic component includes a ceramic element assembly, internal electrodes, and external terminal electrodes. The ceramic element assembly includes a plurality of laminated ceramic layers. The internal electrodes are provided inside the ceramic element assembly. The external terminal electrodes are disposed on the outer surface of the ceramic element assembly. Then, the monolithic ceramic electronic component is arranged on a conductive land of a mounting substrate, and is mounted on the substrate through a conductive bonding material, such as solder.
The monolithic ceramic electronic component mounted on the substrate may undergo external tensile stress. The tensile stress is caused by warping of the substrate or thermal expansion and contraction of the substrate due to changes in external temperature. If the monolithic ceramic electronic component undergoes such stress, the external terminal electrodes may peel off from the ceramic element assembly to cause a so-called open mode failure.
To prevent such an open mode failure, it is a technical challenge to improve the fixing strength of the external terminal electrodes to the ceramic element assembly. For example, Japanese Unexamined Patent Application Publication No. 9-129476 discloses a method to improve the fixing strength. Dummy conductors, which are internal conductors that do not substantially contribute to electrical characteristics, are provided inside the ceramic element assembly, and then metallic bonds between the external terminal electrodes and the dummy conductors are utilized to improve the fixing strength of the external terminal electrodes.
On the other hand, there is a need for further miniaturizing the monolithic ceramic electronic component. When the monolithic ceramic electronic component is miniaturized, an effective area in which the internal electrodes face each other is reduced. Thus, the characteristics tend to decrease. In addition, in a multiterminal monolithic ceramic electronic component, it is necessary to provide a plurality of external terminal electrodes at a narrow pitch. However, an existing method of baking a thick-film paste limits the accuracy of paste coating, and it is difficult to accurately form the external terminal electrodes.
For this reason, a method of directly forming external terminal electrodes by plating has been disclosed. With the above method, thin flat external terminal electrodes may be formed and, as a result, it is possible to increase the effective areas of the internal electrodes. In addition, plating is deposited at the exposed ends of the internal electrodes, such that it is possible to accurately form the external terminal electrodes even at a narrow pitch.
For example, Japanese Unexamined Patent Application Publication No. 2004-327983 discloses that the dummy conductors are also used when the external terminal electrodes are directly formed by plating as described above. Thus, plating metal may be not only deposited at the exposed ends of the internal electrodes but also deposited at the exposed ends of the dummy conductors. This makes it possible to further reliably deposit plating.
In the monolithic ceramic electronic component, it has been determined that forming the dummy conductors inside the ceramic element assembly is a useful technique.
When a monolithic ceramic electronic component including dummy conductors is prepared, internal electrode patterns and dummy conductor patterns are printed on ceramic green sheets. Then, the ceramic green sheets are laminated and sequentially pressure-bonded, and the obtained mother block is stamped.
However, as viewed in a direction in which the ceramic layers are laminated, the internal electrode patterns and the dummy conductor patterns are arranged in an overlapping manner. Thus, the density of the electrode patterns is increased at the overlapping portions. For this reason, the flowability of the ceramic green sheets is inhibited when the mother block is stamped. This may cause a swell (waviness) of the ceramic green sheets in the stamped mother block. Depending on the situation, there is a possibility that the reliability may decrease because of a structural defect, such as delamination.
FIG. 16 shows a partially enlarged cross-sectional view of a monolithic ceramic electronic component 1 that has a deformation due to the above-described swell (waviness).
As shown in FIG. 16, the monolithic ceramic electronic component 1 includes a ceramic element assembly 3 including a plurality of laminated ceramic layers 2. Inside the ceramic element assembly 3, first and second internal electrodes 4 and 5 are alternately arranged in a laminated direction. Ends of the first internal electrodes 4 are exposed at a first side surface 6 of the ceramic element assembly 3. Although not shown in FIG. 16, ends of the second internal electrodes 5 are exposed at a second side surface opposite the first side surface 6 of the ceramic element assembly 3.
In addition, a plurality of first dummy conductors 7 are provided inside the ceramic element assembly 3 so that the first dummy conductors 7 are electrically insulated from the internal electrodes 4 and 5. The ends of the first dummy conductors 7 shown in FIG. 16 are exposed to the first side surface 6 of the ceramic element assembly 3. Although not shown in FIG. 16, second dummy conductors are also provided so that the ends of the second dummy conductors are exposed to the second side surface of the ceramic element assembly 3.
In addition, a first external terminal electrode 8 is disposed on the first side surface 6. The first external terminal electrode 8 is arranged so as to cover the ends of the first internal electrodes 4 and the ends of the dummy conductors 7. Although not shown in the drawing, a second external terminal electrode is disposed on the second side surface of the ceramic element assembly 3.
When stamping a mother block that is prepared to manufacture the monolithic ceramic electronic component 1, if described with reference to a portion shown in FIG. 16, patterns that define the internal electrodes 4 and patterns that define the dummy conductors 7 are arranged in overlapping locations as viewed in the direction in which the ceramic layers 2 are laminated. Thus, as described above, flowability of the ceramic green sheets is inhibited. This easily causes a swell (waviness) of the ceramic green sheets in the stamped mother block. Thus, the deformation as shown in the FIG. 16 occurs.
In addition, as shown in FIG. 17, a process of laminating and sequentially pressure-bonding ceramic green sheets is performed by repeatedly transporting a ceramic green sheet 10 onto a stamping base 11 by a stamping head 9 and laminating and pressure-bonding the ceramic green sheets 10 on the stamping base 11. Here, the plurality of ceramic green sheets 10 that define the desired mother block undergo repeated lamination and pressure-bonding from the bottom, and the initially laminated ceramic green sheets 10 and the conductive patterns, such as the internal electrode patterns and the dummy conductor patterns, undergo a load of pressure-bonding many times. Then, particularly, a pressure concentrates on a portion of the conductive patterns having a high density. In the obtained mother block, a phenomenon occurs in that the conductive patterns at the lower principal surface side, at which the initially laminated ceramic green sheets 10 are located, are expanded by a larger amount than those at the upper principal surface side.
This problem is particularly significant when the external terminal electrodes are formed directly by plating. That is, as the conductive patterns at the lower principal surface side expand, the width of each of the exposed ends of the internal electrodes and dummy conductors differs between the upper principal surface side of the ceramic element assembly and the lower principal surface side of the ceramic element assembly. Then, because of this exposed state, there is a problem in that the external terminal electrodes 12 each have a substantially trapezoidal shape as shown in FIG. 18. FIG. 18 shows a side surface 14 of the ceramic element assembly 13, on which the striped external terminal electrodes 12 are formed in a plurality of lines.
As described above, when the external terminal electrodes 12 each have a substantially trapezoidal shape, it causes, for example, a tombstone defect or a self-alignment defect due to vertical directivity or increases the likelihood that a solder bridge may occur due to close location of the lower portions of the adjacent substantially trapezoidal shapes.